Means and method for making porous bodies of integral structure

ABSTRACT

Porous zinc fabricated bodies are disclosed. The bodies are integral and are essentially of zinc, comprising a continuous network of interconnected pores formed by the zinc. Total porosities of greater than 50 percent are obtained through novel methods utilizing low temperature compaction. The structures are of high strength and good machinability and show interlocking rod shaped particles of zinc.

United States Patent Griffiths et al.

[151 3,655,447 [4 1 Apr-.11, 1972 [54] MEANS AND METHOD FOR MAKING POROUS BODIES. OF INTEGRAL STRUCTURE [72] Inventors: Leonard B. Griffiths, North Reading; Richard H. Krock, Peabody, both of Mass.

P. R. Mallory & Co., lnc., Indianapolis, Ind.

[22] Filed: Sept. 8, 1969 [21] Appl.N0.: 862,616

[73] Assignee 1521 u.s.c1 ..l36/30,75/222 51 Int.Cl. ,.H0lm41/00 58 Field of Search ..136/30, 125, 126, 120; 75/200, 75/201, 211, 214, 222; 29/182, 182.5; 264/41, 42,

[56] References Cited UNITED STATES PATENTS Goodrich et al "vs/20p 3,322,535 5/1967 Rao ..75/201 3,348,976 10/1967 Kelly et al.... ..l36/30 3,413,116 11/1968 Rao ..75/222 3 ,430,031 2/ 1 969 Sunnen ..75/ 200 3,481,789 12/1969 Winsel ..75/222 Primary Examiner-Winston A. Douglas Assistant Examiner-C. F. Lefevour Attorney-Robert Levine [57] ABSTRACT Porous zinc fabricated bodies are disclosed. The bodies are integral and are essentially of zinc, comprising a continuous network of interconnected pores formed by the zinc. Total porosities of greater than 50 percent are obtained through novel methods utilizing low temperature compaction. The structures are of high strength and good machinability and show interlocking rod shaped particles of zinc.

7 Claims, 12 Drawing Figures PATENTEDAPRHIQR I $655,447

SHEET3UF6 l I l l o lo 20 3o 40 so 6o 10 T\N\E M\NUTE S v |v INVENTORS ammo acemmms R c/MED ll. K c/ A ATTORNEY Fig. 9: Mix 6 Structure after firing Mag x 120 .10: Mix 3 As Fig. 9 Mag x120 Fig INVENTORY Aeomzo 8. ate/mm: RIM/9RD b. [620 K fidgw A ORNEY iMEHTEBAPH 11 :972

SHEET Fig. 11: Mix 7 Struct BOF6 ure after firing Mag x 120 Fig. 12: Mix 8 A s Fig. 11 Mag x 120 INVENTOR S ORNEY MEANS AND METHOD FOR MAKING POROUS BODIES F INTEGRAL STRUCTURE This invention relates to porous zinc structures and such, of an integral and self-supporting characteristic, and is particularly directed to such structures useful as electrodes in electrochemical devices.

The need for metal structures of high porosity, i.e., high surface area/volume ratio and of sound structural integrity for application as electrode structures in electrochemical structures and batteries has become very apparent as new devices requiring high rate outputs become manifest.

Accordingly, it is a prime object of this invention to produce an essentially zinc body which has a porosity level of greater than 50%, the pores being uniformly distributed, interconnected and covering a controlled size range from about 20 ,u. to several hundred microns apparent diameter.

It is a further object to produce a body of porous zinc with sufficient strength such that routine handling and simple machining operations may be accomplished without disruption of the body. I

The FIGS. 1, 3, 7, 9, 10, 11 and 12 are microphotographs of the mixes used.

FIGS. 2, 4, 5, 6 and 8' indicate graphs associated as timetemperature profiles of the mixes as fired in a 3-zone furnace.

Since a principal application of such porous bodies is an electrode for mercury electrical batteries, mercury may be added in predetermined quantities during preparation. The quantity involved is no greater than 10% wt.

ln obtaining a porous zinc structure, two requirements become immediately apparent. First, in order to produce bodies of very high porosity, a filler material may be necessary. This porosity, together with the desired range of pore sizes suggests that a coarse particle-size powder should be used.

Secondly, the strength requirement, at such low bulk density determines that, if possible, some fusion of the zinc should be caused to occur during preparation of the bodies. However, this must be carefully controlled in extent since excess densification will otherwise take place.

The objective then is to predeterminally raisethe temperature of the body in a controlled manner in such a way that the liquidus of the nominal composition is just reached. This procedure accomplishes two results. First, the Zn-Hg alloy is far more rapidly homogenized than would be the case if liquidsolid diffusion was used alone. In other words, the relatively low melting point surface is removed quickly. Secondly, a strong bond is formed between adjacent particles. It was found that to accomplish these objectives a 3-zone furnace firing method had to be found. The middle zone was constructed so that it is only 1 inch in length and is controlled such that the thermal profile peaks in the temperature range 405-420 C. Specimens are pulled through the furnace on a small quartz boat equipped with its own thermocouple.

An additional point relates to the ductility of zinc itself. This metal exhibits a maxima in the ductility vs. temperature curve over the range about 1 10 C. to 150 C. It is desired, therefore, to exploit this feature especially in preparing large and complex shaped structures.

Throughout the work, it is a desideration to use very low compacting loads. This is based upon the principle that it is necessary only to obtain sufficient green strength to facilitate handling during subsequent processing. Sufficient strength herein is obtainable at pressures as low, for example, as 1000 psi in the following ways:

1. Production of a mercury-rich surface on each zinc particle, i.e., amalgamation which results in adjacent particles sticking together.

2. Use of zinc powder which contains particles of rod-like morphology. These tend, during mixing, to interlock in the manner of a felted structure.

It is to be noted that the use of high compacting pressures may be disadvantageous and for the following reasons:

1. At the large particle sizes used, fracture of individual particles occurs quite readily. This may lead to weak regions and irregular amalgamation.

2. In the temperature range where ductility is enhanced high pressures lead to relatively large areas of contact. During subsequent fusion, densification may be excessive and difiicult to control (small areas of zinc-zinc contact are preferred.)

3. Die-life is adversely afiected and overall operating costs would be higher.

The teachings shown in assignee's U.S. Pats. Nos. 3,332,535 and 3,337,336 in this field are of interest in the present context. Zinc powder is introduced into an aqueous solution of mercury salt (l-lgCl The mercury salt is thereby decomposed and metallic mercury immediately alloys with the zinc producing a mercury-rich surface thereon. Alternative l-lg salts are HgSO,, Hg(NO )2).

Further, the teachings of F..l. Kelly and F. Przybyla, U.S. Pat. No. 3,348,967, show the removal of ZnO from the surface of the zinc particles in the form of volatile complex. Salts of the ammonium ion (e.g. Nl-LCl) are suitable for the reaction. The complex is removed by sublimation at reduced pressure. As will be shown infra, however, it is possible to obtain strong, porous bodies of zinc by using the present teachings.

While the salt Nl-LCI has been utilized herein, the role is that of a filler with the additional feature of volatilization during the firing operation. This latter feature may tend to keep the structure open. Other fillers are ZnSO,, K Na sO and ZnCO Common to all compositions used to date is the incorporation of mercury, via decomposition of an aqueous solution of l-lgCl However, amalgamation without initial water treatment is within the present purview. Mercury is present to the extent of 5% wt. 10% wt. with respect to zinc.

Mix 1 Zinc powder sieved on 60 mesh sieve-75 gms.

NH CI (Crystal)l0 gms.

HgC] 8 gms. l-lgCl dissolved in about 5 cc of distilled water, zinc added and thoroughly stirred in to facilitate uniform Hg distribution. ZnSO, and Nl-LCl added thoroughly mix.

For small structures, e.g., 0.375 inch dia. 0.060 inch thick. Cold press at 1000 psi eject. Structure is as shown in FIG. 1. For larger structures, e.g., 3 inch X 1% inch X 0.1 inch plate, press at 1000 psi at C., eject.

Fire in a 3-zone furnace in flowing argon or other inert atmosphere. Specimens are placed on a quartz boat which is pulled through the furnace. The time vs. temperature is as shown in FIG. 2.

Mix 2 Zn (on 60 mesh)-75 gms.

ZnS0 -5 gms.

Treat exactly as for Mix number 1 above.

Mix 3 Zn (on 60 mesh)75 gms.

NaCl (Diamond-5 gms.

Crystal brand) HgCl 4 gms. Compacted as Mix number 1. FIG. 3 shows structure. FIG. 4 shows time-temperature profile. Mix 4 Zn (on 60 mesh)75 gms.

Nl-LCl-lO gms.

NaCl--l0 gms.

Subsequent treatment as Mix number 1.

Mix 5 Zn (on 60 mesh)75 gms.

Nl-LCl-IS gms.

ZnSO -10 gms.

l-lgCl -4 gms.

Compactionas previous mixes.

Mix8

Fired in 3-zone furnace in accordance with time-temperature profile of FIG, 5. Mix 6 Zn (on 60 mesh)--75 gms.

3. The type of temperature profiling. 4. Exclusion of fluxinguaugents. 5. High strength, mac eable electrodes.

6. Flat plate electrodes.

NH Cl l gms. 5 7. Low temperature electrodes.

8. l-llgh rate electrodes for battery use. NaCl gms. Th d fin d th f HgC12 4 e present invention e es means an rne ods or mak- Fired in accordance with profile of FIG. 6.

Compaction as previously-See FIG. 7 showing structure. Fired in 3-zone fumace-Profile FIG. 8.

Zn (on 60 mesh)75 gms. NaCl- 1 7 gms.

Treated subsequently as Mix 7.

An additional point is that of dimensional stability. It has been found that with those mixes containing NaCl plates of 3 inch X 1V2 inch X 0.1 inch and donuts" of 0.625 OD X 0.21 inch lD X 0.1 inch may be quite readily raised to well in excess of 410 C. without dimensional instabilities.

All of the bodies heated to fusion in the manner exemplified by the various thermal profiles attached hereto are fully selfsupporting and strong. They may be dropped, struck and machined by drilling and turning without disruption. FIGS. 9- 12 show typical structures after firing.

Electrochemical tests have been performed on samples of The structures were then compared to structures standard with usual cells and their porosities observed. It is to be noted from the table below enhanced efficiency of cells resulted from structures as fabricated above.

It is found from the work done that the following appear to be the major features hereof.

1. Low temperature compaction.

ing porous bodies, but it is intended that the scope of the invention be defined by the appended claims.

Mix 7 10 What is claimed is' me:h) 75 1. A porous structure comprised of an alloy consisting of H l 4 gm zinc-mercury and being of greater than porosity, said gc structure having pores thereof uniformly distributed throughout in a controlled size range from about 20 microns to several hundred microns as determined said alloy containing from 410% mercury, said structure having particles consisting of zinc having rod-like configurations and interlocking in the manner of a felt-like structure, said particles consisting of zinc metal being fired with a thermal profile having a temperature range of 405-420 C., said structure being characterized by good machinability and strength.

2. A zinc-mercury porous structure as in claim 1 in which the particles of zinc are interlocked when the liquidus of the metal is just reached and the rod-like configuration present in uncompacted powder is obtained.

3. A porous structure comprised of an alloy consisting of Zn-Hg of uniform and homogenous distribution characterized by particles of strong bond obtained through quick removal of the low melting point surface and having a porosity due to coarse grain structure of from 20 microns to several hundred microns apparent diameter said alloy containing from 440% mercury, said particles having rod-like configurations and interlocking in the manner of a felt-like structure, said particles consisting of zinc metal being fired with a thermal profile having a temperature range of 405420 C.

4. A zinc-mercury porous structure as in claim 1 wherein compaction pressure is as low as 1000 psi and is characterized by the particles being of rod-like morphology.

5. A zinc-mercury porous structure as in claim 1 fabricated at low compaction pressure, each particle being mercury rich which results in adjacent particles sticking together for good bonding, said particles having a rod-like morphology and a controlled porosity of from 20 microns to several hundred microns and characterized by its morphology to interlock in 2. interlocking of rod-shaped particles of zinc. the manner of a felt-like structure.

Bulk Sp. capac- Sample mix Current, Temp., porosity, Capacity, ity, mA. Efflcieney, number m. "C percent mA. hr. hr./gm. percent Standard Electrodes 6. A mix herein in 1 and urhere NaCl or ZnSO is pressure is as low as 1000 psi and cles being of rod-like morphology.

characterized by the parti- 

2. A zinc-mercury porous structure as in claim 1 in which the particles of zinc are interlocked when the liquidus of the metal is just reached and the rod-like configuration present in uncompacted powder is obtained.
 3. A porous structure comprised of an alloy consisting of Zn-Hg of uniform and homogenous distribution characterized by particles of strong bond obtained through quick removal of the low melting point surface and having a porosity due to coarse grain structure of from 20 microns to several hundred microns apparent diameter said alloy containing from 4-10% mercury, said particles having rod-like configurations and interlocking in the manner of a felt-like structure, said particles consisting of zinc metal being fired with a thermal profile having a temperature range of 405*-420* C.
 4. A zinc-mercury porous structure as in claim 1 wherein compaction pressure is as low as 1000 psi and is characterized by the particles being of rod-like morphology.
 5. A zinc-mercury porous structure as in claim 1 fabricated at low compaction pressure, each particle being mercury rich which results in adjacent particles sticking together for good bonding, said particles having a rod-like morphology and a controlled porosity of from 20 microns to several hundred microns and characterized by its morphology to interlock in the manner of a felt-like structure.
 6. A mix herein as in claim 1 and where NaCl or ZnSO4 is used in the compact.
 7. A porous structure as in claim 3 wherein compaction pressure is as low as 1000 psi and is characterized by the particles being of rod-like morphology. 